This invention relates to a method of producing anode materials for use in electrolysis of various aqueous solutions.
Aqueous solution electrolysis, e.g., electrolytic winning of a non-ferrous metal such as Zn, Cd, Cr, Co, Mn, Cu, and Ni, normally uses a lead alloy, graphite, platinum-plated or platinum-Clad titanium as an insoluble anode material. Insoluble anodes are also used frequently in electroplating, another variation of aqueous electrolysis.
However, the insoluble anode materials that perform fully satisfactorily in electrowinning or electroplating are but few because of various shortcomings.
Where the electrolyte is a sulfuric acid-sulfate solution, e.g., a lead alloy is often used as an insoluble anode. The lead alloy anode releases lead ions dissolving out in trace amounts, and the accumulation of such ions causes the resulting electrodeposit to contain lead which, in turn, gives various unfavorable effects, as is well known in the art.
When the insoluble anode adopted is graphite, it is rapidly consumed due to the breaking down and falling into the electrolyte on account of the oxidation or brittleness of graphite. Thus the life of graphite is short, and the graphite dust falling off can produce contamination of the electrolyte and many other deleterious effects.
Platinum plating or cladding is costly, and platinum is soft enough to be worn with sludge or the like in the electrolyte. The wear combines with electrolytic dissolution to cause early consumption and hence shorter life than expected.
The use of titanium superior in both specific strength and corrosion resistance as an electrode base for insoluble anodes results in the formation of a thick passive film over the titanium surface. This raises the bath voltage until the flow of current becomes totally impossible.
Titanium may occasionally be chosen as an anode where the current density is low. Typical of such unusual cases is the manufacture of electrolytic manganese dioxide, in which pure titanium is an almost exclusive material for the anode at present.
Electrolytic manganese dioxide is used chiefly as the active material of dry cells. It is usually manufactured by electrolysis from an aqueous sulfuric acid-manganese sulfate solution containing from 0.5 to 1.0 mole manganese sulfate and from 0.2 to 0.6 mole free sulfuric acid per liter of the solution.
The aqueous solution upon electrolysis with a direct current on the order of 0.8 A/dm.sup.2 deposits manganese dioxide on the anode. At the stage the deposit has grown enough to a certain extent, it is peeled off and collected as product manganese dioxide. During the process, hydrogen evolves from the cathode.
Titanium has recently come into use as the anode material for the manufacture of electrolytic manganese dioxide. The titanium electrode has such outstanding corrosion resistance, specific strength, and workability that it precludes anode-induced contamination of electrolytic manganese dioxide and yields a quality product.
One problem associated with the use of titanium as the anode for the above process has been the growth of the passive state film on the surface with the increase in current density; it raises the bath voltage accordingly, until the flow of current becomes no longer possible. To overcome the problem it has been necessary to keep the current density within the range around 0.8 A/dm.sup.2
Current density thus has a direct bearing upon productivity in the electrolysis industry. The electrolytic cell employed being the same, the higher the current density the larger would be the scale of production that is made feasible. Also, the output being the same, the electrolytic cell could be made smaller in size as the current density increases, reducing the investment in the electrolytic cell to an economical advantage.
Titanium is used as anodes not merely for the production of electrolytic manganese dioxide but also for other applications. With the latter too the difficulty is that increased current density induces the growth of passive state film on the surface with eventual interruption of current flow. To avoid this, modern practice favors plating of the anodes with the noble metal such as platinum.
However, the treatment using an expensive noble metal casts a heavy financial burden on the manufacturer. It thus presents a major obstacle in the way of the extensive commercial acceptance of the plated anodes.
With these in view, this invention is aimed at providing a method of producing titanium alloy anodes which can carry more current at higher density than conventional titanium anodes can.